Our progress has centered on the evaluation of the minimal molecular and cellular events required for elicitation of anti-viral and anti-H-2 CTLs. We have been able to incorporate purified viral and/or H-2 antigens into unilamellar phospholipid vesicles and elicited specific secondary CTLs with these defined stimulator fractions. Stimulation of secondary anti-viral CTLs requires only purified viral-specific surface glycoproteins and H-2 antigens incorporated into the same lipid matrix. To elicit secondary anti-H-2 CTLs the purified H-2 antigens incorporated into a lipid matrix alone is sufficient. These results restrict the molecular requirements for a cell to be an effective stimulator of CTLs.